1. Field of the Invention
This invention relates to a laser driving device suitable for use in an optical information recording apparatus and to an optical information recording apparatus using such a laser driving device.
2. Related Background Art
Various forms such as a disk-like form or a cardlike form, are known as types of a medium on which information is recorded by the use of light and from which the recorded information is read out. Of these, an optical information recording medium formed into the shape of a card (hereinafter referred to as an "optical card") expects a great demand as a medium of large recording capacity, which is compact, light in weight and convenient for carrying.
Referring to FIG. 1 of the accompanying drawings, which is a schematic plan view of such an optical card 101, reference numeral 102 designates an information recording area, reference numeral 103 denotes information tracks, reference numerals 104 and 104' designate track selecting areas, and reference numeral 105 denotes the home position of a light beam spot.
Referring now to FIG. 2 of the accompanying drawings, which shows the construction of an apparatus for recording and reproducing information on the optical card, reference numeral 106 designates a motor for driving the optical card 101 in the direction of the double-headed arrow, reference numeral 1 denotes a semiconductor laser, reference numeral 108 designates a collimator lens for collimating the light from the semiconductor laser 1, reference numeral 109 denotes a beam splitter, reference numeral 110 designates an objective lens, reference numeral 111 denotes a coil for tracking, reference numeral 112 designates a coil for focusing, reference numerals 113 and 114 denote condensing lenses, the reference numerals 115 and 116 designate photoelectric converting elements, reference numeral 117 denotes a tracking control circuit, and reference numeral 118 designates a focusing control circuit. On the basis of a tracking signal and a focusing signal detected by the photoelectric converting elements 115 and 116, an electric current is supplied to the coil 111 for tracking and the coil 112 for focusing in accordance with the commands from the control circuits 117 and 118, whereby the objective lens 110 is moved to effect auto tracking and auto focusing. The reference numeral 119 denotes a system controller for controlling recording-reproducing apparatus, and reference numeral 120 designates a group of various control signals output from the system controller 119. Although not shown, signals other than the signals 120 are also output from the controller 119. The reference numeral 121 denotes an optical head, and reference numeral 122 designates a driving motor for moving the optical head in the direction indicated by arrow u shown in FIG. 1.
An electric current, modulated in conformity with an information signal input from a terminal 124, is supplied from a laser driving device 123 to the semiconductor laser 1.
In the above-described optical information recording-reproducing apparatus, recording of information is accomplished by condensing the light beam emitted from the semiconductor laser 1 into a minute spot of about several .mu.m by the objective lens 110, applying the minute spot to the optical card 101 and forming a pit differing in shape or reflectance from the surroundings. Also, reproduction of the recorded information is accomplished by applying a semiconductor laser light, of weaker light power than that during recording, to the recording portion of the optical card, and detecting any variation in the intensity of the reflected light from a recording layer.
Thus, during recording, when information is written, the semiconductor laser driving device must change between a laser light of high output and laser light of low output at a high speed in conformity with a recording signal and must cause them to be emitted from the semiconductor laser. During reproduction, when information is read, the laser driving device must to cause a laser light of low output to be output at a predetermined value.
However, the threshold current of the semiconductor laser varies due to temperature or lapse of time and therefore, even for the same driving current value, the light output value fluctuates due to the ambient temperature or lapse of time. During recording, this results in a factor which causes excess and deficiency in the state of writing onto the recording medium and reduces the reliability of the recorded information, and during reproduction, this results in a factor which aggravates the S/N ratio of the reproducing signal.
From the above-noted point, it is necessary, in the semiconductor laser driving device used in an optical information recording-reproducing apparatus, that laser light of stable high output and low output, respectively, be supplied.
FIG. 3 of the accompanying drawings shows the construction of an example of the semiconductor laser driving device heretofore used. In this example, there are provided two laser drivers 6 and 9 for driving a semiconductor laser 1. During reproduction, only the output of the driver 6 for reproduction is supplied to the semiconductor laser 1, and during recording, the output of the driver 6 for reproduction is held and the output of the driver 9 for recording is superposed on the output of the driver 6, whereby a driving current for recording is obtained. The reference numeral 4 designates a sample hold circuit which holds when a L sample hold control signal WZ is at level H, and samples when the sample hold control signal WZ is L.
The operations during reproduction and recording will hereinafter be described in detail.
During reproduction, only the output current of the driver 6 for reproduction is supplied to the semiconductor laser 1. The driving current for reproduction is stabilized by inputting, to an error comparing circuit 5, a monitor voltage obtained by voltage-converting the photocurrent of a photodiode (PD) 2 for monitoring by a current-voltage (I/V) converter 3 and the output of a reference voltage setting circuit 7 for setting a laser power value for reproduction, and controlling the driver 6 for reproduction so that the monitor voltage becomes equal to the reference voltage.
During recording, the output of the I/V converter 3 is held by the sample hold circuit 4 immediately before a recording data signal WD is output, whereby the power for reproduction is fixed. The output current of the driver 9 for recording, controlled on the basis of a recording power reference voltage setting circuit 10, is switched on or off by a change-over switch 8 in accordance with the recording data signal WD and a recording signal current is superposed on a reproduction driving current, whereby a recording power is obtained. However, in this case, the control, which as during reproduction, the monitored value of the light power by the photodiode 2 for monitoring is compared with the reference voltage and corrected, is not effected. Also, the fixing of the power for reproduction by this holding is released when return is made to the reproducing mode.
The timing charts of the sample hold control signal WZ, the recording data signal WD and the light output power corresponding to the above-described instances are shown in FIG. 4 of the accompanying drawings.
In the prior-art system described above, the recording power P.sub.W cannot be stabilized for a variation in the ambient temperature and a variation with lapse of time. This is true, because the differential efficiency of the semiconductor laser (an inclination above the threshold value of the forward current-light output characteristic) fluctuates due to variation in temperature and variation with lapse of time.
Details thereof will now be described with reference to FIG. 5 of the accompanying drawings. In FIG. 5, a indicates the initial characteristic of the semiconductor laser, and b shows the characteristic after the initial characteristic a has been subjected to a variation with lapse, of time and a variation in temperature. In contrast with characteristic a, characteristic b is increased in threshold current I.sub.th and reduced in differential efficiency. It will be seen from this figure that even if the reproducing power P.sub.R changes from the characteristic a to the characteristic b, the current (I.sub.R .fwdarw.I.sub.R ') is kept constant by an increase in by an increase in the reproduction driving current, while the recording power changes from P.sub.W to P.sub.W ' and is not stabilized, because the recording power current I.sub.W superposed on the reproduction power current I.sub.R ' remains unchanged.